For instance, on Special:BlankPage:
That’s a 1920×1080px screenshot, but the page container only takes up 1712×445px, and as a result the bottom of the sidebar, e. g. the “printable version” link, is not available.
This breaks the AC/DC browser tests (build #397). They were still passing yesterday (build #396), so presumably this is due to some change that was merged today.